The present invention relates to an antenna arrangement, the arrangement comprising an electrically thin microwave phasing structure including a support member and a reflective means for reflecting microwaves within a frequency operating band supported by said supporting member. The support member at a distance from the reflective means supports an arrangement of electromagnetic-loading structures.
Furthermore, methods are provided for designing and manufacturing electrically thin microwave phasing structures for electromagnetically emulating desired reflective surfaces and focussing elements of selected geometry.
U.S. Pat. No. 4,905,014 discloses an electrically thin microwave phasing structure for electromagnetically emulating a desired reflective surface of selected geometry over an operating frequency band. The microwave phasing structure comprises a support matrix and a reflective means for reflecting microwaves within the frequency-operating band. The support matrix supports the reflective means. An arrangement of electromagnetically loading structures is supported by the support matrix at a distance from the reflective means, which can be less than a fraction of the wavelength of the highest frequency in the operating frequency range. The electromagnetically loading structures are dimensioned, oriented, and interspaced from each other and disposed at a distance from the reflective means, as to provide the emulation of the desired reflective surface of selected geometry. Another aspect of the present invention is the use of the electrically thin microwave phasing structure for electromagnetically emulating a desired microwave focusing element of a selected geometry.
Other phasing structures are also known, e.g. through U.S. Pat. Nos. 4,656,487; 4,126,866; 4,125,841; 4,017,865; 3,975,738; and 3,924,239.
In xe2x80x9cDesign of Millimeter Wave microstrip Reflectarraysxe2x80x9d, By David M. Pozar et al, IEEE Transactions on Antennas and Propagation, Vol. 45, No., Feb. 2, 1997, pages 287-295, a theoretical modelling and practical design of a millimeter wave reflect arrays using microstrip patch elements of variable size are discussed.
One major problem related to antennas according to above-mentioned documents in general and the arrangement according to U.S. Pat. No. 4,905,014 in particular, is the cross coupling problem between the crossing elements of the cross-shaped or similar dipoles in one plane.
Flat parabolic surface technology is based on a dipole pattern over a ground plane with a dielectric material there between.
Preferably, the spacing between the dipoles is chosen to avoid grating lobes, i.e. it must be less than half a wavelength.
Experiments have shown that the width of the dipoles not only affects the bandwidth of the reflector but also the phase shift and phase gap of the reflected wave. The phase gap is in an interval in the full 360 degree phase range to which phase shift is not possible.
The length of the dipoles affects the reflected phase shift. This is due to the fact that a dipole""s characteristic impedance is dependent on its length. A dipole is said to be resonant when the reactive part of the impedance is zero, i.e., when the input admittance is infinite. For a single dipole this occurs when the dipole length is approximately a half wavelength.
A small dipole width results in a small phase gap but the dipole shift becomes more sensitive of frequency; decreasing the phase gap results in an undesired decrease in the bandwidth. The phase shift also depends on the incremental angle.
The impedance Z of an antenna determines the efficiency with which it acts as a conductor between the propagation medium to the feeder and the transmission line connecting it to the system with which it operates. If there is an array of dipoles it is necessary to consider not only the self impedance of each dipole but also the mutual coupling between the dipoles. The mutual impedance increases when the distance between the dipoles decreases. It is therefore desired to maximize the distance between the elements.
It is assumed that the equivalent circuit of a single dipole contains three parallel loads: a loss conductance GL, a transmission admittance YT and a dipole susceptance B. The loss conductance is due to the finite conductivity of the dipole, which in turn is due to losses in the conductor and the dielectric material. Depending on the incremental angle, the dipole excites an electromagnetic wave with different phases because the dipole radiation scattered from the dipole to the ground plane has different path lengths through the dielectric layer. This effect is illustrated by the admittance YT. A dipole is said to be resonant when the reactive part of the input impedance is zero, i.e. the input admittance is infinite. For a single dipole, this occurs when the dipole length is approximately a half wavelength.
When the antenna is a linear array of dipoles, the equivalent circuit of the dipole has to be modified. The mutual impedance between the dipoles has to be considered, whereby a mutual admittance Ymn between dipoles m and n, where  less than  greater than  n and self-impedance of dipole m when m=n, is added in parallel to above-mentioned loads. However, the problem is even more complex where two-dimensional arrays of dipoles are employed.
One object of the present invention is to provide a solution to the above-mentioned problem and provide an enhancement to the antenna reflectors known through to the prior art, which is commercially usable in wide range of applications.
Another object of the present invention is to provide a reflector device in an antenna arrangement, which is easy to produce and configure for several types of applications.
Yet another object of the present invention is to provide a flat antenna reflector with more compact dipole configuration. Preferably, longer dipoles can also be arranged.
One additional object of the present invention is to provide a small, inexpensive, easily modified reflector replacement in radio-link arrangements, preferably microwave link antennas, in a cellular network, which further is simple to assemble for providing different types of lobe configurations, such as point to point and point to multipoint and which replaces parabolic reflectors.
The invention also has as an object to provide an antenna reflector, which can be mounted flat on a carrying surface and which can be arranged to shape the main lobe, change the direction of the beam, be offset fed and have low cross polarization.
Moreover, the antenna reflector according to the present invention reflects very little of the cross polar radiation and it reflects the radiation that has a frequency outside the specified bandwidth very poorly, provides a low main beam RCS (Radar to Cross Section) for the frequencies outside the bandwidth which the antenna is designed for.
Therefore, the electromagnetic-loading structures are arranged on at least two substrate layers in at least two planes.
Preferably, the dipoles are arranged in an angel on one side of said substrate on each layer, which allows longer dipoles.
In one embodiment the dipoles have a substantially cross-shaped configuration having substantially vertical and horizontal dipole elements arranged in different planes, which allows circular polarization.
Preferably, the dipoles have different sizes and/or shapes, which result in different lobe shapes and/or directions, and also different frequency reflections.
The arrangement can be configured as a reflector in a center-fed broad side antenna, a center-fed antenna with a tilted main lobe, an offset-fed broad side antenna, a Point-to-Point or Point-to-Multipoint antenna.
Preferably, the dipoles are arranged on different substrates, but they may also be arranged on different sides of a substrate.
The invention also refers to an antenna at least comprising one electromagnetic feeding arrangement and reflector arrangement, which comprises an electrically thin microwave phasing structure including a support member, supported by said supporting member a reflective means for reflecting microwaves within a frequency operating band and a phasing arrangement of electromagnetic-loading structures supported by said support matrix. The electromagnetic-loading structures are interspaced from each other and disposed at a distance from said reflective means by said support matrix so as to provide said emulation of said desired reflective surface of selected geometry. Moreover, the electromagnetic-loading structures are arranged on at least two substrate layers in at least two planes.
In one embodiment the antenna comprises different feeders for different planes.
In still a further embodiment the antenna comprises a further reflector facing said reflector arrangement, which is arranged to reflect vertically or horizontally polarized electromagnetic waves and said further reflector is arranged to rotate said vertical or horizontal polarization to horizontal or vertical polarization.
The invention also concerns a method of producing an antenna reflector. The method comprises the steps of: determining characteristics of an antenna employing the reflector; calculating a distance between the feeder and each dipole with respect to the input characteristics; calculating a phase shift for the dipoles; and using said calculated phase shift for calculating the length of the dipoles. The characteristics include antenna size, type, frequency band, feeder type, feeder size etc. For calculating said phase shift an analyzing procedure is used, which analyses: a microstrip dipole surrounded by an infinite number of identical dipoles; dual layer dichroic structures, which consist of two parallel metallic screens (gratings) separated by one/several dielectric layers; and a single grating surrounded by a number of dielectric layers that are considered to be electrically close to the grating.